Dooku is a human male of the aristocratic house of Serenno, a Jedi Master who left the Order and became Darth Tyranus, political head of the Separatist movement. He was introduced in Star Wars: Episode II Attack of the Clones (2002) and played by Christopher Lee.
The Lost Twenty
Dooku is one of the very few Jedi to voluntarily resign from the Order — the Lost Twenty — and the fiction takes that seriously as a political act rather than a moral failure. His critique of the Republic is essentially correct: it is corrupt, the Senate is bought, and the Jedi have become its enforcement arm. The tragedy is that being right about the diagnosis made him easy to recruit for the disease.
Jedi Lost
Cavan Scott's Dooku: Jedi Lost (Del Rey, 2019) is the essential canon text and, unusually, an audio drama first — a full-cast production later published as a script book. It works backwards through his life: apprentice, master to Qui-Gon Jinn, aristocrat estranged from a family he was taken from as an infant, and a man accumulating reasons.
Legends
Star Wars Legends covered similar ground across the Clone Wars multimedia project and James Luceno's Labyrinth of Evil, with a rather more straightforwardly villainous Dooku. Current canon is markedly more interested in the resignation.
Reading note
Read him next to Qui-Gon Jinn. Master and apprentice reach nearly the same verdict on the Order, and go in opposite directions with it — which is the cleanest statement of the prequel era's actual argument.
